-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KPHI 221945
AFDPHI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
345 PM EDT WED MAY 22 2013

.SYNOPSIS...
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A BERMUDA HIGH PRESSURE SYSTEM WELL OFF THE EAST COAST WILL CONTINUE
TO PUMP WARM AND MOIST AIR ACROSS THE MIDDLE ATLANTIC REGION TONIGHT
THROUGH THURSDAY. A LOW PRESSURE SYSTEM NEAR CHICAGO THIS EVENING
WILL MOVE NORTHEAST ACROSS THE GREAT LAKES AND THEN DOWN THE SAINT
LAWRENCE VALLEY INTO FRIDAY. A SECOND LOW PRESSURE SYSTEM WILL
DEVELOP OFF NEW ENGLAND FRIDAY AND REMAIN ACROSS THE WATERS INTO
SATURDAY. HIGH PRESSURE WILL MOVE IN SUNDAY AND REMAIN INTO EARLY
NEXT WEEK.
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&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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AN 18Z SURFACE ANALYSIS PLACED LOW PRESSURE ACROSS THE GREAT LAKES
ALONG A FRONTAL BOUNDARY THAT WAS DRAPED FROM NEW ENGLAND TO THE
GREAT LAKES. HIGH PRESSURE WAS CENTERED EAST OF NORTH CAROLINA. A
TROUGH WAS ACROSS VIRGINIA. AN UPPER AIR ANALYSIS SHOWED A TROUGH IN
THE NORTHERN MISSISSIPPI VALLEY AND GREAT LAKES, WITH A STRONG
CLOSED LOW IN THE PACIFIC NORTHWEST.

THE RATHER MOIST AIRMASS HELPED TO PRODUCE LOTS OF LOW CLOUDS,
HOWEVER THESE CONTINUE TO ERODE FROM WEST TO EAST. THESE MAY HANG ON
THOUGH ACROSS OUR NORTHEAST ZONES AND ALONG THE COAST GIVEN THE
MOIST SOUTHERLY FLOW. A WARM FRONT SHOULD CONTINUE TO LIFT NORTHWARD
THROUGH TONIGHT, AND THIS BOUNDARY WAS LOCATED IN NEAR FAR NORTHEAST
NEW JERSEY THIS AFTERNOON. OVERALL, A WARM AND MUGGY NIGHT IS IN
STORE ACROSS THE ENTIRE CWA. THIS SHOULD ALSO LEAD TO ADDITIONAL
STRATUS TO DEVELOP OVERNIGHT ALONG WITH SOME FOG. FOR NOW, ADDED IN
PATCHY FOG AS WE ARE THINKING THE MOISTURE SHOULD BE MOSTLY FOCUSED
INTO THE FORM OF CLOUDS.

THE MAIN FOCUS IS THE CONVECTIVE POTENTIAL FOR THE REMAINDER OF THE
AFTERNOON AND INTO TONIGHT. THE MODEL GUIDANCE OVERALL IS KEYING IN
ON OUR NORTHWEST ZONES WHERE THESE AREAS WILL BE CLOSER TO THE
BETTER LARGE SCALE FORCING AS A FRONTAL BOUNDARY AND SURFACE LOW ARE
DRAPED ACROSS THE GREAT LAKES. THERE IS PLENTY OF INSTABILITY IN
PLACE OWING TO THE HIGHER DEW POINTS. HOWEVER, THE LACK OF FOCUSED
FORCING MAY KEEP THE CONVECTIVE COVERAGE ON THE LOW SIDE. THE ENTITY
TO WATCH IS AN INCOMING SHORT WAVE THAT COULD TEAM UP WITH A LEE
SIDE TROUGH, THAT WILL INITIATE CONVECTION TO OUR WEST AND
SOUTHWEST. OTHER THAN THAT, PERHAPS AN ISOLATED CELL IS ABLE TO POP
UP INLAND INTO THIS EVENING. WE THEREFORE CONTINUED WITH SLIGHT CHC
TO LOWER CHC POPS, BUT THEN LOWERED THESE FOR AWHILE TONIGHT BEFORE
INCREASING SOME IN THE WEST TOWARD MORNING.

THE STRONGER WESTERLY FLOW ALOFT IS FARTHER NORTH AND MOSTLY FROM
THE GREAT LAKES REGION AND ACROSS NEW YORK STATE. SOME OF THE HIGH
RESOLUTION MODELS INDICATE CONVECTION DEVELOPING ACROSS CENTRAL
PENNSYLVANIA THEN TRACKING NORTHEASTWARD. THIS INITIATION COULD BE
ENHANCED SOME BY THE TERRAIN AND A LEE SIDE TROUGH AS A SHORT WAVE
APPROACHES, AND IF THIS CONVECTION ARRIVES IT COULD CONTAIN LOCALLY
STRONG WINDS AND HAIL MOSTLY ACROSS OUR NORTHWESTERN ZONES LATER
THIS AFTERNOON AND THIS EVENING. THE OTHER AREA IS IN WESTERN
VIRGINIA AS THIS ACTIVITY TRACKS NORTHEASTWARD. IT DOES HAVE A WAYS
TO GO YET BEFORE REACHING OUR REA. THE PW VALUES ARE AROUND 1.25
INCHES, THEREFORE ANY STORM THAT IS ABLE TO DEVELOP WILL BE CAPABLE
OF PRODUCING LOCALLY VERY HEAVY RAIN. A MENTION WAS CONTINUED IN THE
HAZARDOUS WEATHER OUTLOOK.

AS FOR LOW TEMPERATURES, WE MAINLY USED AN EVEN GFS/NAM MOS BLEND.
THE HOURLY TEMPERATURE AND DEW POINT GRIDS INCORPORATED SOME OF THE
LAMP/LAV GUIDANCE TO ASSIST IN TRENDS.
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&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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THIS LOOKS TO BE AN ACTIVE DAY WITH SHOWERS AND SOME THUNDERSTORMS
BECOMING MORE NUMEROUS. THIS IS IN RESPONSE TO THE LARGE SCALE
LIFT INCREASING AS AN UPPER-LEVEL TROUGH APPROACHES ALONG WITH THE
ARRIVAL OF A COLD FRONT AND WEAK SURFACE LOW. THERE MAY BE SOME
SHOWERS /PERHAPS SOME THUNDER/ ONGOING AS THE DAY STARTS AS
INCREASING LIFT OVERSPREADS THE AREA. THIS SHOULD BE MAINLY ACROSS
THE INLAND AREAS. WE ARE ANTICIPATING AN ABUNDANCE OF CLOUDS TO BE
PRESENT DURING THE DAY, WITH AREAS OF STRATUS AND PERHAPS FOG
GRADUALLY LIFTING SOME. THE PRESENCE OF THE CLOUDS ADDS UNCERTAINTY
TO THE AMOUNT OF INSTABILITY THAT WILL BE PRESENT. THE FORECAST
SOUNDINGS INDICATE THE AIRMASS WILL BE RATHER BUOYANT, THEREFORE
SHOWERS MAY TEND TO DEVELOP FAIRLY QUICKLY EVEN WITH INITIALLY
WEAKER LIFT.

THERE IS THE CHC FOR SOME STRONGER CONVECTION, ESPECIALLY IF POCKETS
OF INCREASED INSTABILITY CAN OCCUR. AS STRONGER SOUTHWESTERLY FLOW
OVERSPREADS THE AREA, ANY ROBUST UPDRAFTS THAT CAN BE SUSTAINED WILL
BE CAPABLE OF PRODUCING GUSTY WINDS AND HAIL PROVIDED STRONGER
INSTABILITY CAN BE OBTAINED. SINCE THERE IS SOME UNCERTAINTY
REGARDING THE STRENGTH AND ORGANIZATION OF CONVECTION, NO ENHANCED
WORDING WAS INCLUDED IN THE FORECAST. THE PW VALUES ARE FORECAST TO
BE ABOUT 1.50 TO 1.75 INCHES, THEREFORE LOCALLY VERY HEAVY RAIN
COULD OCCUR. AS OF NOW, MAINLY SOME LOCALIZED POOR DRAINAGE FLOODING
COULD RESULT. A MENTION IS IN THE HAZARDOUS WEATHER OUTLOOK. THE
POPS INCREASE FROM WEST TO EAST DURING THE DAY, ALTHOUGH WE DID SLOW
THE INCREASE DOWN SOME ACROSS THE EASTERN ZONES. THE ACTIVITY MAY BE
HELD INLAND FOR AWHILE AS LARGER SCALE LIFT TAKES LONGER TO GET MORE
ORGANIZED ACTIVITY GOING ALONG THE COAST.

AS FOR TEMPERATURES, MOSTLY A GFS/NAM MOS BLEND WAS USED.
TEMPERATURES ARE EXPECTED TO BE LOWER THAN TODAY GIVEN MORE CLOUDS
THROUGHOUT THE DAY COUPLED WITH INCREASING SHOWERS AND STORMS.
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&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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A PERIOD OF WET WEATHER AT THE BEGINNING OF THE EXTENDED PERIOD AS
THE UPPER FLOW BEGINS TO SHARPEN AND A SFC/UPPER LOW IS SHOWN
DEVELOPING ACROSS THE NEW ENGLAND REGION THU NIGHT INTO FRIDAY. THIS
LOW IS SHOWN ONLY MOVING SLOWLY EASTWARD INTO SATURDAY. THIS
REPRESENTS A CHANGE IN THE LONGER RANGE GFS/ECMWF MODELS OVER THE
PAST FEW RUNS. THEY WERE SHOWING MORE RAPID CLEARING ON PREVIOUS
RUNS. WE HAVE ACCORDINGLY INCREASED CLOUD/POP GRIDS FOR THESE
PERIODS OVER PREVIOUS VALUES. TEMPERATURES WILL DROP BELOW NORMAL
AFTER THE COLD FRONT PASSAGE THU NIGHT AND TEMPERATURES WILL STAY
BELOW NORMAL INTO THE WEEKEND. HIGHS GENERALLY IN THE 60S FRI/SAT
AND LOWS MOSTLY IN THE 40S/50S FRI AND SAT NIGHTS.


THE REMAINING PART OF THE EXTENDED PERIOD LOOKS TO FEATURE PLENTY OF
DRY WEATHER. THE ONLY PERIOD WHEN PCPN IS IN THE GRIDS IS FRO NEXT
WED WHEN A SLGT CHC IS IN THERE. THE HIGH PRESSURE SYSTEM TO THE
WEST ON SUNDAY WILL CREST OVER THE REGION MONDAY NIGHT/TUESDAY AND
THEN CONTINUE EASTWARD THROUGH THE MIDDLE OF NEXT WEEK. TEMPERATURES
WILL BE A LITTLE BELOW NORMAL SUNDAY WITH HIGHS IN THE UPPER 60S/LOW
70S IN MANY AREAS. READINGS WILL MODERATE MONDAY THROUGH WEDNESDAY
WITH HIGHS BY WEDNESDAY REACHING BACK INTO THE UPPER 70S AND FEW LOW
80S. HUMIDITY LEVELS WILL BE LOW/MODERATE SUNDAY AND MONDAY AND THEN
THE HUMIDITY WILL SLOWLY INCREASE INTO THE MIDDLE OF NEXT WEEK.
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&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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THE FOLLOWING DISCUSSION IS FOR KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY AND SURROUNDING AREAS.

REST OF THIS AFTERNOON...ANY LINGERING LOCALIZED MVFR CEILINGS
LIFTING AND SCATTERING OUT. OTHERWISE, VFR. THERE IS THE CHANCE OF A
FEW SHOWERS/THUNDERSTORMS, WITH THE GREATEST POTENTIAL TO BE WELL
WEST OF KPHL. A MENTION WAS NOT INCLUDED DUE TO A LOW POTENTIAL AT
THIS TIME. SOUTHWEST WINDS GENERALLY 10-15 KNOTS.

TONIGHT...VFR, THEN IFR/MVFR CEILINGS AND VISIBILITIES ARE
FORECAST TO DEVELOP. THE TIMING OF THIS IS OF LOW CONFIDENCE. THERE
COULD BE A SHOWER OR THUNDERSTORM ESPECIALLY IN THE EVENING NORTH
AND WEST OF KPHL, THEN ANOTHER CHANCE MAINLY TOWARD DAYBREAK.
SOUTH-SOUTHWEST WINDS GENERALLY UNDER 10 KNOTS.

THURSDAY..IFR/MVFR CEILINGS AND/OR VISIBILITIES. THERE IS A CHANCE
FOR SOME IMPROVEMENT TO VFR, HOWEVER GIVEN INCREASING SHOWERS WE DID
NOT FORECAST THIS AT THIS TIME. SHOWERS AND SOME THUNDERSTORMS ARE
FORECAST TO BECOME MORE NUMEROUS DURING THE DAY WITH REDUCED
VISIBILITIES. LOCALLY HEAVY RAIN AND GUSTY WINDS COULD ACCOMPANY
SOME OF THE ACTIVITY. SOUTH-SOUTHWEST WINDS GENERALLY 10-15 KNOTS.

OUTLOOK...
THU NIGHT THRU SAT MORNING...LOWER CLOUDS/SHOWERS AND LOWER
VSBYS/CIGS POSSIBLE.
SAT AFTERNOON THRU MONDAY...MOSTLY VFR.
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&&

.MARINE...
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A SOUTHERLY FLOW WILL CONTINUE THROUGH THURSDAY AHEAD OF A COLD
FRONT AND SURFACE LOW PRESSURE. THIS FLOW IS FORECAST TO INCREASE
DURING THIS TIME, HOWEVER FORECAST SOUNDINGS INDICATE A SHARP
LOW-LEVEL INVERSION IS IN PLACE DUE TO MUCH WARMER AIR OVER THE
COOLER WATERS. THIS WILL TEND TO KEEP THE STRONGER WINDS FROM ALOFT
FROM MIXING DOWN TO THE SURFACE, ALTHOUGH THE GFS IS A BIT STRONGER
WITHIN THE INVERSION LEVEL. WE ARE THINKING THAT THE WINDS GENERALLY
REMAIN UNDER 25 KNOTS, HOWEVER THE SEAS ARE FORECAST TO BUILD WITHIN
THE SOUTHERLY FLOW. IT APPEARS WAVEWATCH IS RUNNING ABOUT A FOOT TO
HIGH SO FAR, THEREFORE WE DID UNDERCUT THE GUIDANCE SOME. AS A
RESULT, THE SMALL CRAFT ADVISORY FOR THE OCEAN ZONES /MAINLY FOR
SEAS/ REMAINS IN EFFECT STARTING EARLY THIS EVENING AND CONTINUES
THROUGH THURSDAY. SOME FOG SHOULD BE MAINTAINED INTO THURSDAY,
ESPECIALLY ACROSS OUR NORTHERN WATERS. SHOWERS AND SOME
THUNDERSTORMS ARE EXPECTED THURSDAY, MOST NUMEROUS IN THE AFTERNOON,
AND SOME STORMS COULD PRODUCE LOCALLY GUSTY WINDS AND VERY HEAVY
RAIN.

OUTLOOK...
THE SCA FLAG HAS BEEN EXTENDED INTO THU NIGHT FOR NOW. THE SLOWER
MOVING FRONT AND DEVELOPING LOW PRESSURE TO THE NORTH MAY RESULT
IN THE SCA FLAG BEING EXTENDED FURTHER IN TIME. IMPROVING WEATHER
BY THE SECOND HALF OF THE WEEKEND.
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&&

.PHI WATCHES/WARNINGS/ADVISORIES...
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PA...NONE.
NJ...NONE.
DE...NONE.
MD...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 6 AM EDT FRIDAY FOR ANZ450>455.
-- End Changed Discussion --

&&

$$
SYNOPSIS...O`HARA
NEAR TERM...GORSE
SHORT TERM...GORSE
LONG TERM...O`HARA
AVIATION...GORSE/O`HARA
MARINE...GORSE/O`HARA